1. Field of the Invention
The invention relates to a method of determining the receptivity of wireless signals in a broadcast system and to a receiver for performing the operations necessary for the reception.
The invention may be especially practiced in a broadcast system, and, more particularly, in a common frequency system, with the wireless signals embracing, for example, the transmitting stations, program signals and/or program varieties which can be received. For when receiving radio transmitting stations with mobile receivers, for instance in an automotive vehicle, it is desirable to determine the programs which can be received at any given receiving location. Such programs may include, for instance, traffic reports or information relating to a traffic conduction system.
2. The Prior Art
Present day analog frequency modulated (FM) transmission methods react sensitively to variations in field strength and to multi-path reception the effects of which may only partially be reduced, for instance, by elaborate change-over strategies to so-called alternative frequencies which are transmitted as components of the radio data signals (RDS). Defining a station by means of a mobile receiver has hitherto required elaborate measures. Thus, complex circuit arrangements are required, often even including two receiving sections, reference transmitters and/or lists of alternative frequencies stored in the receiver. The latter is required for switching, wherever possible without delay and inaudibly, to alternative frequencies in case a program tuned in on a mother station can either not be received at all or poorly only. To obtain, depending upon the actual receiving location, data about receivable programs is possible to a limited extend only with existing systems, such as, for example, the aforementioned RDS. Moreover, the scanning operations for finding receivable transmitters and their identification require a relatively long time.
A method utilizing the radio data signals of at least three stationary transmitters for passive evaluation to define a location with a mobile wireless receiver is known from German patent specification 4,107,116. The publication states that the method offers the possibility of linking the defined position coordinates of the mobile wireless receiver with route-specific and/or geographic identification signals of traffic reports transmitted via the RDS signal over the traffic message channel. In this manner, only those reports which are significant to the instantaneous location are selected from the transmitted reports; all others are faded out. The disadvantage of such a system is that the operator has to preselect the given program variety, such as, in the present example, the traffic report, so that prior to his selection he does not know whether he will receive anything at all, or what it will be. Hence, it will take some time after one or more searching operations until the operator will actually receive the desired information.
To achieve a qualitatively excellent wireless audio transmission corresponding to the quality standard offered by digital storage media (for example DAT), a standard was developed for a terrestrial digital transmission method, known as DAB (digital audio broadcasting). One of the essential characteristics of the DAB method is the common frequency operation of the transmitters employed for a receiving area, with all of the transmitters being connected in a frequency and phase locked relationship and the modulation contents of the individual carriers being identical for all transmitters. From German Patent Specification 4,223,194 it is known that a receiver suitable for DAB may simultaneously be utilized for defining a location, so that no additional receivers are required. Additional transmitters are also not required.
Moreover, German Patent Specification 4,222,877 describes how regionally or locally different data may be transmitted in a DAB network with technical means, without interfering with the common frequency transmission of locally identical data. The transmission of regionally different data is specifically carried out by additional transmission from the transmission station of individual carrier frequencies which are preferably transmitted in a time slot of a transmission window which is also utilized for synchronizing the receiver. The receiver, for performing receiving operations, is provided with a memory and an indicator, additional data being stored in the memory by means of a data record associated with each transmission station. They may either be displayed on the indicator, or they should enable the operator of the receiver to improve the quality of the reception, for instance, by changing the receiving parameters. The specification only discloses how regionally limited data can be transmitted with technical means within a DAB network.